Sigma-delta converters are useful in such applications as communications systems and wireless Local Area Networks (LAN). These converters provide high dynamic range and flexibility in converting low bandwidth input signals. The general principle of the sigma-delta architecture is to make rough evaluations of a signal and to measure, integrate, and compensate for errors in the evaluation.
A conventional first order sigma-delta Analog-to-Digital Converter (ADC) has two primary components: a sigma-delta modulator and a digital filter. An input signal X is fed into the sigma-delta modulator and to an amplifying structure, such as an integrator, resonator, and/or passive structure (capacitor, resistor, inductor, and so forth), referred to as an integrator herein. The integrator distributes a converter quantization error, or quantization noise, such that it is very low in the band of interest according to well known techniques. The integrator outputs a signal that is input to a comparator. The output of the comparator is directed to a feedback Digital-to Analog Converter (DAC), which outputs an analog approximation signal Q back to be combined with the input signal X. The analog signal Q is subtracted from the analog input signal X in an attempt to reduce in-band quantization noise, and to force the average of the signal Q to be equal to the input signal X. The resulting output signal is sent through the comparator and provided as an output signal from the sigma-delta modulator to a digital filter, which may be a decimation filter.
The integrator, as described above, may be replaced with a filter that has a transfer function of H(s)=1/s. The quantizer may be modeled as a noise source, or a summation node, with a noise contribution of q. The quantization noise q is fed back using a feedback loop to be combined with the input signal. In summary, the error signal containing the noise q passes through a filter with high in-band frequency gain and high out-band frequency attenuation, which shapes the quantization noise spectrum. At least some of the quantization noise q is shifted to out-band frequencies, thus generating an output signal with mainly an in-band frequency of interest.
While the invention may be modified, specific embodiments are shown and explained by way of example in the drawings. The drawings and detailed description are not intended to limit the invention to the particular form disclosed, and instead the intent is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the claims.